Project Summary Screening with in vivo phage display of constrained peptide libraries (comprising about different 108 sequences) has been employed as a new approach to identify specific molecular signatures present in the brains of transgenic mice that model Alzheimer's Disease (AD). The transgenic animal model used for this screening is the J20 tg hPPP, that encodes amyloid precursor protein with two mutations associated with human AD. Three peptides that specifically recognize the hippocampus (the primary site of the disease) in the J20 mice have been identified. These peptides accumulate from an intravenous injection in the hippocampal area of the diseased brains and not in the brains of normal littermates. The lead peptide has been shown to bind to activated astrocytes in the neurovascular unit of AD hippocampus. This peptide also recognizes human AD-derived samples: Endothelial cells differentiated from human AD iPS cells bind the peptide, whereas cells prepared similarly from a control individual do not, and the peptide also binds to brain sections from a patient with sporadic AD. This application proposes to identify the molecular targets (receptors) for these peptides. These molecules, because they distinguish AD from normal brain, may reveal new features of the pathogenesis of AD. They should also provide new opportunities for diagnostic and therapeutic targeting of AD. The approach proposed here significantly differs from the current state of the art in the field, as current approaches have generally focused on targeting misfolded or aberrant proteins such as amyloid plaques. In contrast, the first three peptides from the phage screens recognize specific changes in the vascular and extravascular neuronal tissue of AD hippocampus, and do so prior to any plaque development. !